Understanding Cambial Behaviour The key to wood quality

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Understanding Cambial Behaviour The key to
wood quality
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• A brief history
• Terminology
• Dormancy and reactivation
• Growth of derivatives and wall formation
• Pitting and plasmodesmata

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• A brief history

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Nehemiah Grew (1641-1712)
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• Grews drawing of elm (detail)

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Charles Francois Brisseau-Mirbel

• Proposed that cambium was a tissue rather than a
  sap (1808)

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Mirbels (1827) diagram of elm (from Larson, 1994)
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Cambial cell theories

• Hartig (1853)- Back to back theory
• Phloem initial
• Xylem initial

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Cambial cell theories

• Sanio (1863)- Single initial theory

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Cambial cell theories

• Raatz/Mischke (1892) Multiple initial theory

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• Oblique orientation of plane of division
• Kinoplasmic fibres (microtubules)
• Kinoplasmosomes (phragmoplast

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• From Bailey (1923)
• Anticlinal pseudotransverse division
• Transverse division

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Length of cambium/cambial age (from Bailey 1923)

• A Conifer or vessel-less dicot. (12 species)
• B Less specialised dicot. (10 species)
• C Highly specialised dicot. (10 species)
• D Dicot. with storeyed cambium (10 species)

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Vacuolation in cambial cells (Bailey 1930)
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• Pinus radiata
• Active cambium
• Nomenclature
• Cambium?
• Cambial Zone?

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Butterfield (1975) IAWA Bulletin 13 14

• Cambium a multiseriate zone of periclinally
• dividing cells lying between the differentiating
• secondary xylem and phloem, with a distinct
• initial capable of both periclinal and anticlinal
• divisions lying somewhere within each radial file
• of cells

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• Cambium according to Butterfield

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Schmid (1976) IAWA Bulletin 51-59

• Cambium equivalent to the initiating layer
• Cambium applied to the entire differentiating
  region might lead to the conception that the
  cambium is a multiseriate layer of initials

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• Cambium according to Schmid
• ?

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The difficulty of identifying the initial means
the terms have been used interchangeably
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• Pinus radiata
• Mid-winter
• A slowly dividing meristem

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• Pinus radiata
• Mid-winter
• Cambium
• Butterfield
• Schmid ?

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Humpty DumptyFrom Through the Looking Glass
and what Alice found there by Lewis Carroll
When I use a word, Humpty Dumpty said in rather
a scornful tone, it means just what I choose it
to mean neither more nor less
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• Can an initial ever be identified with certainty?

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• Aesculus hippocastanum
• February
• Cells appear similar across the
• cambium

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• Boundary parenchyma phloem
• side
• Boundary parenchyma xylem
• side

Identifying an initial
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• A. hippocastanum TEM
• Boundary parenchyma
• (phloem side)

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• A. hippocastanum TEM
• Phloem cells in suspended or
• slow development

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• A. hippocastanum TEM
• Fusiform Initial

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• A. hippocastanum TEM
• Boundary parenchyma
• xylem side

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• Sieve element/companion cell
• pair in a state of arrested
• development
• Initial

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• Boundary parenchyma
• Companion cell precursor
• Sieve/element precursor

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• Phloem boundary cell
• Previous seasons phloem
• Sieve tube member
• Companion cells
• 9 February

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• Dormancy and reactivation

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• Dormant Cambium
• Fragmented vacuome

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• Storage materials in dormant fusiform cells
• Spherosomes (lipids)
• Protein bodies
• Thick cell walls

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9 February Fusiform initial Ray
initial
Starch
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Cambial reactivation in Aesculus

• Activity can be detected in the cytoplasm of
  cambial zone cells long before any signs of
  activity are displayed by the tree.

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• Active dictyosome in a
• boundary layer cell of
• dormant cambium
• 23 February

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• Developing and mature coated vesicles
• 8 March

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• Reactivation (16 March)
• Expanding phloem precursors
• Fusiform initial
• Boundary parenchyma

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• Dividing phloem mother cell (16 March)
• New tangential wall

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• 13 April
• Boundary parenchyma
• Cytoplasm confined to a thin parietal layer
• Boundary parenchyma

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• 23 April
• Developing phloem cells
• Dividing initial
• Xylem mother cell
• New xylem elements

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• Typically in the Reading area, Aesculus bud-break
  occurs in late March, with leaves fully emerged
  by late April
• Xylem formation appears to begin coincidentally
  with leaves beginning to export photosynthate

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Reactivation sequence

• Larson (1994)
• Xylem production first 26 species
• Phloem production first 21 species
• Simultaneous production 10 species

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Observations are inconsistent between authors

• Acer pseudoplatanus, Quercus rubra , Pinus
  sylvestris and Vitis vinifera appear in the list
  of xylem reactivators and phloem reactivators
• In the Pinaceae
• Pinus halepensis and rigida are xylem
  reactivators
• Pinus banksiana, resinosa, and strobus (five
  authors) are phloem reactivators
• Picea excelsa, rubens and rubra are xylem
  reactivators,
• Picea abies is a phloem reactivator
• Picea glauca a simultaneous reactivator.

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Phloem production in Aesculus

• Phloem annual growth rings marked by boundary
  parenchyma
• The number of phloem cells in each file is
  similar to the number of over-wintering
  precursors
• All the phloem for the season is produced at the
  beginning of the season

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• Pinus radiata
• Active cambium producing both xylem and phloem
  throughout the season

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• Growth of derivatives and wall formation

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Cell enlargement
Quercus robur
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• 20 April
• Developing xylem cells
• Boundary parenchyma
• Previous years latewood fibre

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• Cell tip growing between fibres

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Enlarging vessel element Boundary
parenchyma
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• Developing fibres are compressed and files of
  cells distorted by vessel enlargement

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Perforation plates
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Secondary wall formation

• This the classic representation of the wall of a
  cell that has all possible wall layers
• the MLPS1S2S3 HTW wall zones

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Cell wall Layers
Middle lamella
Primary wall
S1
S2
S3
Helical thickening (tertiary layer)
Cell lumen
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• Earliest stages of cellulose deposition forming
  the S1 layer

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Microtubules and cellulose orientation
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Tubulin in a fusiform cambial cell of Aesculus
(B), and developing fibres (C, D, E)
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Tubulin in developing fibres in Populus
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Tubulin in tension wood fibres of Populus
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The significance of microfibril angle

• The relationship between MFA and axial stiffness
  according to Cave (1968)
• There is a 5 fold increase in stiffness when MFA
  shifts from 40 to 10 degrees

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Corewood in a 125-year Old Tree

• Juvenile wood (large microfibril angle)
• Mature wood
• (small microfibril angle)

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Corewood in a 25-year Old Tree

• Juvenile wood
• Mature wood

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The Problem of High Microfibril Angle

• Corewood is too flexible to be used as high grade
  timber
• Any improvement that would reduce the amount of
  low grade timber would result in significant
  financial gain for the producer and result in
  more efficient use of forest land

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Consequences for the Tree

• High microfibril angle in corewood makes young
  trees flexible and able to withstand high winds
• Only small reductions in angle may be feasible
  without affecting survivability
• These may, however, still give significant
  increases in the quality of corewood

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• Pitting and plasmodesmata

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Formation of pits
Aesculus hippocastanum vessel wall
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Sorbus aucuparia
Cambium pit fields
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Pit fields in enlarging fibres
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Pinus radiata
Cambium pit fields
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Pit fields in enlarging tracheids
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• Interference contract micrograph of a TLS through
  the radial wall of a tracheid of Pinus radiata

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Pinus radiata

• Pit fields in radial walls of enlarging tracheids

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Developing torus
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Pinus radiata

• Beginning of formation of the torus
• and pit border

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Functional and aspirated bordered pits
P. radiata
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Vessel pitting seen from the middle lamella in
Aesculus
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Vessel-vessel wall in Aesculus hippocastanum
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Plasmodesmata in radial wall of cambium in
Aesculus
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Fibre-fibre pit in Aesculus
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Pits in a tangential wall between ray parenchyma
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Plasmodesmata in pit fields

• Absent
• Vessels and tracheids (conifer and angiosperm)
  to any other cell type
• Present
• Fibres to fibres and parenchyma
• Parenchyma to parenchyma and fibres

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Plasmodesmata in developing vessel walls of
hybrid aspen
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• Microtubules and pit formation

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Tubulin in a developing Aesculus vessel
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Tubulin in young vessel elements in Aesculus
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• Plasmodesmata in pit membranes in some members
  of the Rosaceae

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Sorbus aucuparia RLS 2µm thick Plasmodesmata
appear in section as black spots
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Pit fields in developing fibres of Sorbus
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Conclusions

• Microscopy reveals that cambial behaviour varies
  between species
• A knowledge of ultrastructural changes during
  differentiation of xylem is essential to
  understanding wood formation processes
• The cytoskeleton and plasmodesmata are important
  factors in the control of xylem differentiation
• Cambial behaviour ultimately governs wood
  structure and quality

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The End